The present invention pertains generally to the synthesis of phosphazene oligomers and polymers and in particular to the dialkylation of these oligomers and polymers.
Alkylation of phosphazene oligomers is of particular importance for improvements in thermal stability, glass-transition temperature, degree of crystallinity and melt behavior of polyphosphazenes polymerized from these oligomers. These polymers are becoming increasingly important because of the flexibility, flame retardation, and resistance to ultraviolet light at temperatures above 200.degree. C. Furthermore, the decomposition temperature of these polymers, being in excess of 200.degree. C., constitutes a significant improvement over organic polymers.
It has been determined that the hydrogen-phosphorous bond is the least stable bond in these compounds. Consequently, attempts have been made to replace the hydrogen atom with other substituents. Limited success has been met in substituting alkoxy or aryl groups on the phosphorus atom. An example of an alkoxy substitution on the phosphorus atom is found in H. R. Allcock et al., J. Amer. Chem. Soc., 99 6095-6 (1977). In C. W. Allen et al., Inorg. Chem. 7, 2177-83 (1968), a method for synthesizing aryl-substituted phosphazene fluoride is described.
Attempts to directly bond alkyl groups to the phosphorus atom of polyphosphazenes through carbon-phosphorus bonds have, on the whole with one exception, been unsuccessful. Generally, they proceed by a reaction of a Grignard or organo-lithium reagent with poly(dichlorophosphazenes). These attempts result in cleavage reactions due to the preference of these reagents to degrade the phosphazene skeleton in preferance to replacement of the halogen. See, for example, C. F. Liu, R. L. Evans, U.S. Pat. No. 3,169,933 issued in 1965 and J. R. Callum et al., J. Polym. Sci., part A-1, 6 3163-5 (1968).
The only successful alkylation of the phosphorus atom of a polyphosphazene is reported in P. J. Harris and H. R. Allcock, J. Amer. Chem. Soc., 100:20 6512-3 (1978). This method proceeds by reacting a hindered alcohol with the organo-copper intermediate formed by the reaction of a poly(dichlorophosphazene), a Grignard agent, and [n-Bu.sub.3 PCuI].sub.4. The product so produced has only a single alkyl group on the phosphorus atom, leaving a thermally and hydrolytically unstable hydrogen-phosphorus bond. While these polymers had a reasonably high decomposition temperature, approximately 200.degree. C., the hydrolytic instability of the remaining phosphorus-hydrogen bonds limited these polymers for uses in totally dry environments.